This invention relates to the diffusion bonding of reactive metals, such as titanium alloys, and which may be subsequently formed into structures by superplastic forming, and is particularly concerned with the removal of entrapped air from the surfaces of such metallic workpieces prior to diffusion bonding thereof, to prevent reaction of the air with the hot reactive metals prior to diffusion bonding.
A number of alloys exhibit superplasticity and are capable of being subjected to superplastic forming to produce parts of predetermined shapes. Superplasticity is the capability of a material to develop unusually high tensile elongation with reduced tendency toward local necking during deformation. Prior to such superplastic forming, diffusion bonding of the metal workpieces is carried out to bond the workpieces in certain preselected areas, to permit superplastic forming to be carried out in the unbonded areas of the workpieces.
Structures have been successfully produced from a number of titanium-based alloys by the Superplastic Forming/Diffusion Bonding (SPF/DB) process. Such structures are producible because many titanium alloys exhibit the two essential physical properties required for SPF/DB, namely ability to be diffusion bonded and superplasticity.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause comingling of atoms at the joint interface.
One category of metals which can be diffusion bonded and which have superplasticity characteristics are termed "reactive" metals. These include alloys of titanium, zirconium, and the refractory metals.
U.S. Pat. No. 3,927,817 discloses a method for fabrication of structures in which metal blanks, preferably of a titanium alloy, are joined at selected areas by diffusion bonding at elevated temperatures and pressures, and then subjected to superplastic forming to form a desired structure. The metal blanks are first treated at selected areas with a stopoff material, such as yttria, boron nitride, graphite, or alumina, to prevent bonding at such treated areas during diffusion bonding. During superplastic forming the metal blanks are expanded at the treated (unbonded) areas into contact with shaping members by increasing the internal pressure, preferably with an inert gas, thus forming an expanded structure of a desired shape, essentially in a single operation.
Thus, after the bonds between adjacent metal blanks are formed during diffusion bonding, inert gas pressure, such as argon or helium, is applied to the interior network to superplastically form the unbonded portions of the adjacent metal sheets.
However, prior to and during diffusion bonding, air (oxygen and nitrogen) will react with hot, reactive metals and alloys such as titanium alloys, resulting in surface contamination which prevents or inhibits the ability to join the metallic workpieces by solid state diffusion bonding.
Diffusion bonding temperatures can vary from 1450.degree. F. to about 1850.degree. F., e.g. about 1700.degree. F. for 6Al-4V titanium alloy, and bonding pressure can vary from about 100 psi to about 2000 psi or more, usually about 150 psi to about 600 psi. If argon is employed to expel air prior to and during diffusion bonding, it has been found that at temperatures from about 800.degree. F. and up to the diffusion bonding temperature, a good diffusion bond is not obtainable.
U.S. Pat. No. 3,937,387 discloses a method of diffusion bonding of aluminum containing components, including a method of preparing aluminum containing surfaces for such welding techniques wherein the aluminum surfaces are freed of any aluminum oxide coating and are coated with a polymeric sealer such as polystyrene which can be thermally removed leaving essentially no residue, the polymeric sealer being removed in a substantially oxygen-free environment and the aluminum components then being brazed or diffusion bonded without the use of a flux to remove oxide coating.
U.S. Pat. No. 3,486,016 discloses a process for producing carbon-free joints preparatory to welding light metal members which are initially coated with a polymer capable of depolymerization such as polybutylene to produce a gaseous monomer at welding temperature and thereafter heating the members to a welding temperature in an atmosphere free of oxygen and finally welding the members while heating in the atmosphere.
U.S. Pat. No. 3,591,917 discloses a method for joining refractory and reactive surfaces to themselves or to other surfaces by depositing a layer of a titanium-indium alloy between the joinable surfaces, and heating under vacuum to evaporate the indium to leave a layer of titanium joining the surfaces.
U.S. Pat. Nos. 3,047,917; 3,821,018 and 3,325,432 are illustrative of a coating employed either as a corrosion-oxidation prevention layer or as a release agent. In this respect polytetrafluoroethylene (TEFLON) is also known for use as a lubricant for tools and dies.
It is an object of the present invention to provide an improved process for removal of any entrapped air from the surfaces of reactive metals prior to diffusion bonding, by means of an inert gas other than nitrogen.
Another object of the invention is the provision of procedure for readily removing entrapped air from the bondline between reactive metal workpieces, particularly titanium alloy workpieces, prior to diffusion bonding, by producing inert gases heavier than air and forcing the air and such inert gases out of such bondline, to produce a diffusion bond of high quality.
A still further object is the provision in the above noted procedure of means for producing such inert gases at temperatures above 800.degree. F., and below the diffusion bonding temperature, such inert gases having a density greater than air, but less than argon, so that a mixture of the entrapped air and such inert gases can be swept away from the contacting surfaces during diffusion bonding by means of argon.